The present invention relates generally to radio or wireless communications and, more particularly, relates to a transceiver capable of simultaneously receiving and transmitting signals over at least two bands.
Wireless communication systems are an integral component of the ongoing technology revolution. Mobile radio communication systems, such as cellular telephone systems, are evolving at an exponential rate. In a cellular system, a coverage area is divided into a plurality of xe2x80x9ccellsxe2x80x9d. A cell is the coverage area of a base station or transmitter. Low power transmitters are utilized, so that frequencies used in one cell can also be used in cells that are sufficiently distant to avoid interference. Hence, a cellular telephone user, whether mired in traffic gridlock or attending a meeting, can transmit and receive phone calls so long as the user is within a xe2x80x9ccellxe2x80x9d served by a base station.
Mobile cellular systems were originally developed as analog systems. After their introduction for commercial use in the early 1980s, mobile cellular systems began to experience rapid and uncoordinated growth. In Europe, for example, individual countries developed their own systems. Generally, the systems of individual countries were incompatible, which constricted mobile communications within national boundaries and restricted the market for mobile equipment developed for a particular country""s system. In 1982, in order to address this growing problem, the Conference of European Posts and Telecommunications (CEPT) formed the Groupe Spxc3xa9ciale Mobile (GSM) to study and develop a set of common standards for a future pan-European cellular network. It was recommended that two blocks of frequencies in the 900 MHz range be set aside for the system. The initial goals for the new system included international roaming ability, good subjective voice quality, compatibility with other systems such as the Integrated Services Digital Network (ISDN), spectral efficiency, low handset and base station costs, and the ability to support new services and a high volume of users.
One of the initial, major decisions in the development of the GSM standard was adoption of a digital, rather than an analog, system. As mentioned above, analog systems were experiencing rapid growth and the increasing demand was straining the capacity of the available frequency bands. Digital systems offer improved spectral efficiency and are more cost efficient. The quality of digital transmission is also superior to that of analog transmission. Background sounds such as hissing and static and degrading effects such as fadeout and cross talk are largely eliminated in digital systems. Security features such as encryption are more easily implemented in a digital system. Compatibility with the ISDN is more easily achieved with a digital system. Finally, a digital approach permits the use of Very Large Scale Integration (VLSI), thereby facilitating the development of cheaper and smaller mobile handsets.
In 1989, the European Telecommunications Standards Institute (ETSI) took over responsibility for the GSM standards. In 1990, phase I of the standard was published and the first commercial services employing the GSM standard were launched in 1991. It was also renamed in 1991 as the Global System for Mobile Communications (still GSM). After its early introduction in Europe, the standard was elevated to a global stage in 1992 when introduced in Australia. Since then, GSM has become the most widely adopted and fastest growing digital cellular standard, and is positioned to become the world""s dominant cellular standard. With (currently) 324 GSM networks in operation in 129 countries, GSM provides almost complete global coverage. As of January 1999, according to the GSM Memorandum of Understanding Association, GSM accounted for more than 120 million subscribers. Market research firms estimate that by 2001, there will be more than 250 million GSM subscribers worldwide. At that time, GSM will account for almost 60% of the global cellular subscriber base, with yearly shipments exceeding 100 million phones.
Two frequency bands of 25 MHz were allocated for GSM use. As illustrated in FIG. 1a, the 890-915 MHz band is reserved for transmission or xe2x80x9cuplinkxe2x80x9d (mobile station to base station), and the 935-960 MHz band is reserved for reception or xe2x80x9cdownlinkxe2x80x9d (base station to mobile station). An extra ten MHz of bandwidth was later added to each frequency band. The standard incorporating this extra bandwidth (two 35 MHz bands) is known as Extended GSM (EGSM). In EGSM, the transmission band covers 880-915 MHz and the receiving band covers 925-960 MHz (FIG. 1b). The terms GSM and EGSM are used interchangeably, with GSM sometimes used in reference to the extended bandwidth portions (880-890 MHz and 925-935 MHz). Sometimes, the originally specified 890-915 MHz and 935-960 MHz bands are designated Primary GSM (PGSM). In the following description, GSM will be used in reference to the extended bandwidth (35 MHz) standard.
Due to the expected widespread use of GSM, capacity problems in the 900 MHz frequency bands were anticipated and addressed. ETSI had already defined an 1800 MHz variant (DCS or GSM 1800) in the first release of the GSM standard in 1989. In DCS, the transmission band covers 1710-1785 MHz and the receiving band covers 1805-1880 MHz (FIG. 1c). In the United States, the Federal Communications Commission (FCC) auctioned large blocks of spectrum in the 1900 MHz band, aiming to introduce digital wireless networks to the country in the form of a mass market Personal Communication Service (PCS). The GSM service in the US is known as PCS or GSM 1900. In PCS, the transmission band covers 1850-1910 MHz and the receiving band covers 1930-1990 MHz (FIG. 1d).
Regardless of which GSM standard is used, once a mobile station is assigned a channel, a fixed frequency relation is maintained between the transmit and receive frequency bands. In GSM (900 MHz), this fixed frequency relation is 45 MHz. If, for example, a mobile station is assigned a transmit channel at 895.2 MHz, its receive channel will always be at 940.2 MHz. This also holds true for DCS and PCS; the frequency relation is just different. In DCS, the receive channel is always 95 MHz higher than the transmit channel and, in PCS, the receive channel is 80 MHz higher than the transmit channel.
The architecture of one implementation of a GSM network 20 is depicted in block form in FIG. 2. GSM network 20 is divided into four interconnected components or subsystems: a Mobile Station (MS) 30, a Base Station Subsystem (BSS) 40, a Network Switching Subsystem (NSS) 50 and an Operation Support Subsystem (OSS) 60. Generally, MS 30 is the mobile equipment or phone carried by the user; BSS 40 interfaces with multiple MSs 30 and manages the radio transmission paths between the MSs and NSS 50; NSS 50 manages system switching functions and facilitates communications with other networks such as the PSTN and the ISDN; and OSS 60 facilitates operation and maintenance of the GSM network.
Mobile Station 30 comprises Mobile Equipment (ME) 32 and Subscriber Identity Module (SIM) 34. ME 32 is typically a digital mobile phone or handset. SIM 34 is a memory device that stores subscriber and handset identification information. It is implemented as a smart card or as a plug-in module and activates service from any GSM phone. Among the information stored on SIM 34 are a unique International Mobile Subscriber Identity (IMSI) that identifies the subscriber to system 20, and an International Mobile Equipment Identity (IMEI) that uniquely identifies the mobile equipment. A user can access the GSM network via any GSM handset or terminal through use of the SIM. Other information, such as a personal identification number (PIN) and billing information, may be stored on SIM 34.
MS 30 communicates with BSS 40 across a standardized xe2x80x9cUmxe2x80x9d or radio air interface 36. BSS 40 comprises multiple base transceiver stations (BTS) 42 and base station controllers (BSC) 44. A BTS is usually in the center of a cell and consists of one or more radio transceivers with an antenna. It establishes radio links and handles radio communications over the Um interface with mobile stations within the cell. The transmitting power of the BTS defines the size of the cell. Each BSC 44 manages multiple, as many as hundreds of, BTSs 42. BTS-BSC communication is over a standardized xe2x80x9cAbisxe2x80x9d interface 46, which is specified by GSM to be standardized for all manufacturers. The BSC allocates and manages radio channels and controls handovers of calls between its BTSs.
The BSCs of BSS 40 communicate with network subsystem 50 over a GSM standardized xe2x80x9cAxe2x80x9d interface 51. The A interface uses an SS7 protocol and allows use of base stations and switching equipment made by different manufacturers. Mobile Switching Center (MSC) 52 is the primary component of NSS 50. MSC 52 manages communications between mobile subscribers and between mobile subscribers and public networks 70. Examples of public networks 70 that MSC 52 may interface with include Integrated Services Digital Network (ISDN) 72, Public Switched Telephone Network (PSTN) 74, Public Land Mobile Network (PLMN) 76 and Packet Switched Public Data Network (PSPDN) 78.
MSC 52 interfaces with four databases to manage communication and switching functions. Home Location Register (HLR) 54 contains details on each subscriber residing within the area served by the MSC, including subscriber identities, services to which they have access, and their current location within the network. Visitor Location Register (VLR) 56 temporarily stores data about roaming subscribers within a coverage area of a particular MSC. Equipment Identity Register (EIR) 58 contains a list of mobile equipment, each of which is identified by an IMEI, which is valid and authorized to use the network. Equipment that has been reported as lost or stolen is stored on a separate list of invalid equipment that allows identification of subscribers attempting to use such equipment. The Authorization Center (AuC) 59 stores authentication and encyrption data and parameters that verify a subscriber""s identity.
OSS 60 contains one or several Operation Maintenance Centers (OMC) that monitor and maintain the performance of all components of the GSM network. OSS 60 maintains all hardware and network operations, manages charging and billing operations and manages all mobile equipment within the system.
The GSM transmitting and receiving bands are divided into 200 kHz carrier frequency bands. Using Time Division Multiple Access techniques (TDMA), each of the carrier frequencies is subdivided in time into eight time slots. Each time slot has a duration of approximately 0.577 ms, and eight time slots form a TDMA xe2x80x9cframexe2x80x9d, having a duration of 4.615 ms. One implementation of a conventional TDMA frame 80 having eight time slots 0-7 is illustrated in FIG. 3a. 
In this conventional TDMA framework, each mobile station is assigned one time slot for receiving data and one time slot for transmitting data. In TDMA frame 80, for example, time slot zero has been assigned to receive data and time slot four has been assigned to transmit data. The receive slot is also referred to as the downlink slot and the transmit slot is referred to as the uplink slot. The remaining slots are used for offset, control, monitoring and other operations. This framework permits simultaneous reception by as many as eight mobile stations on one frequency and simultaneous transmission by as many as eight mobile stations on one frequency.
In recently proposed GSM standards (phase 2+), a multi-slot mode of operation is defined. In multi-slot operation, a mobile station transmits and/or receives in several time slots within each TDMA frame (as opposed to the configuration of frame 80, wherein there is only one receive and one transmit timeslot per frame). Two types of mobile stations having multislot capabilities divided into eighteen classes have been defined. A type 1 mobile station is not required to transmit and receive at the same time. An example of a TDMA frame 85 used by a type 1 mobile station having multi-slot capabilities is shown in FIG. 3b. TDMA frame 85 has two receive time slots 0 and 1 and two transmit time slots 3 and 4. Many other receive and transmit slot assignments are, of course, possible. For a type 1 mobile station, though multiple time slots are used to receive and transmit, the mobile station is still not required to transmit and receive within the same time slot.
A type 2 mobile station has the capability to receive and transmit within the same time slot. A TDMA frame 90 suitable for use in conjunction with a type 2 mobile station is illustrated in FIG. 3c. TDMA frame 90 transmits in slots 4-6 and receives in slots 1-4. Hence, a mobile station using this framework would be required to both receive and transmit in slot 4. Generally, as the number of time slots used for receiving and transmitting increases, and as the number of time slots in which both receiving and transmitting occurs increases, the class designation also increases. In the highest class, a class 18 type 2 mobile station, all eight time slots of each TDMA frame are configured to both receive and transmit.
Under current GSM standards using the conventional TDMA framework, the mobile station transmitter and receiver can easily share a frequency resource. In TDMA frame 80, for example, when going from receive mode to transmit mode, there are three time slots during which the required transmit frequency can be generated from a shared frequency resource (i.e. a local oscillator). This provides a great deal of freedom in the transceiver architecture and frequency generation scheme. Under the proposed multi-slot standards, however, there is less flexibility for sharing frequency resources. In the type 1 mobile station framework of FIG. 3b, for example, there is only one time slot available to change from the receive frequency to the transmit frequency. For a type 2 mobile station with multi-slot capability, using a TDMA framework such as that of FIG. 3c, the design challenges are further intensified. The required receive and transmit frequencies must be generated simultaneously.
Moreover, as described above, there are currently three GSM frequency bands defined. With the proliferation of wireless handset usage showing now signs of slowing down, it is likely that additional bands will be defined in the future. Hence, GSM mobile stations intended for global usage should have multi-band capability. A multi-band design magnifies the already substantial challenge of designing a cost efficient transceiver having type 2 multislot capability.
In accordance with the purpose of the invention as broadly described herein, there is provided a cost-effective, multi-band transceiver that is capable of type 2 multi-slot operation. The transceiver has a frequency generation architecture that permits reception and transmission within the same time slot on any given channel within any of the GSM frequency bands.
In one embodiment of the present invention, a transceiver having simultaneous transmission and reception capability is provided. It comprises a first local oscillator LO1 that generates a signal having a frequency fLO1, and a second local oscillator LO2 that generates a signal having a frequency fLO2. A receiver receives a signal having a frequency fRx, wherein fRx is one of the sum or the difference of frequencies fLO1 and fLO2. The receiver comprises an image rejection mixer that mixes frequencies fLO1 and fLO2 to generate a demodulating signal at the receive frequency fRx, and a quadrature down converter that mixes the demodulating signal with the receive signal to produce baseband xe2x80x9cIxe2x80x9d and xe2x80x9cQxe2x80x9d signals. A transmitter transmits a signal having a frequency fTx, wherein fTx is equal to fRx minus a comparison frequency fCF. The transmitter comprises a voltage-controlled oscillator (VCO) that generates the transmit signal, a mixer that mixes the transmit signal with the signal from the first local oscillator LO1 to produce an IF signal having a frequency fIF, a quadrature mixer modulates the IF signal with baseband xe2x80x9cIxe2x80x9d and xe2x80x9cQxe2x80x9d signals, a first divider that divides fIF by an integer M down to fCF, a second divider that divides fLO2 by an integer N down to fCF, and a phase detector that compares the phases of the signals output by the first and second dividers and outputs a control voltage to the first VCO.
In another embodiment of the present invention, a multi-band transceiver for receiving and transmitting signals within a selected one of at least two frequency bands is provided. Transmission and reception occurs simultaneously and within the same time slot of a TDMA frame. The transceiver includes a first local oscillator LO1 that selectively oscillates within a bandwidth corresponding to the selected frequency band and outputs a signal having a frequency fLO1. A second local oscillator LO2 selectively oscillates at a frequency corresponding to the selected frequency band and outputs a signal having a frequency fLO2. A receiver receives a signal having a frequency fRx and mixes the signals from the first and second local oscillators to generate a demodulating signal having a frequency fRx, wherein fRx=fLO1xc2x1fLO2. A transmitter having a loop architecture includes a VCO that generates a signal having a transmit frequency fTx equal to fRx minus a comparison frequency fCF, and a mixer that mixes the signal from the first local oscillator with the transmit signal to generate an IF signal having a frequency fIF=fLO2xc2x1fCF. The transmitter also includes a quadrature mixer that modulates the IF signal with baseband xe2x80x9cIxe2x80x9d and xe2x80x9cQxe2x80x9d signals, and a phase detector that compares the phases of the IF signal and the signal from the second local oscillator and outputs a control voltage to the VCO.
The present invention also provides a method for simultaneously transmitting and receiving signals. The method comprises the following steps:
(a) generating two local oscillation (LO) signals;
(b) receiving a receive signal;
(c) demodulating the receive signal using the LO signals;
(d) generating a transmit signal;
(e) modulating the transmit signal using one of the LO signals;
(f) aligning the phase of the transmit signal using the other of the LO signals; and
(g) transmitting the modulated transmit signal.
Steps (a)-(c) are performed repetitively and simultaneously with steps (d)-(g).
In another method according to the present invention, signals are transmitted and received within the same time slot of a TDMA frame. The method comprises the following steps:
(a) generating a first local oscillation signal LO1 having a frequency fLO1;
(b) generating a second local oscillation signal LO2 having a frequency fLO2;
(c) receiving a receive signal having a frequency fRx;
(d) mixing signals LO1 and LO2 to generate a demodulating signal having a frequency fRx;
(e) mixing the demodulating signal and the receive signal to generate baseband xe2x80x9cIxe2x80x9d and xe2x80x9cQxe2x80x9d signals;
(f) generating a transmit signal having a frequency fTx;
(g) mixing the transmit signal and the LO1 signal to generate an IF signal having a frequency fIF;
(h) modulating the IF signal with baseband xe2x80x9cIxe2x80x9d and xe2x80x9cQxe2x80x9d signals;
(i) dividing the frequency of the IF signal fIF down to a comparison frequency fCF;
(j) dividing the frequency of the LO2 signal fLO2 down to the comparison frequency fCF;
(k) comparing the phases of the divided IF and LO2 signals and adjusting the frequency of the transmit signal if necessary; and
(l) transmitting the modulated transmit signal.
Objects and advantages of the present invention include any of the foregoing, singly or in combination. Further objects and advantages will be apparent to those of ordinary skill in the art, or will be set forth in the following disclosure.